Purpose: The aim of this study was to ascertain the possibility of using the Small Animal Radiation Research Platform (SARRP) in pre-clinical experiments using X-ray FLASH dose rates.
Methods: A Small Animal Radiation Research Platform (SARRP) was used to deliver 45 Gy using a Standard dose rates of 0.24 Gy/s and a FLASH dose rate of 45 Gy/s. At 3.5 cm SSD the field was 26 mm in diameter. Only the half of the beam area closest to the anode was used because the exposure achieved uniform dose distribution and a high dose rate. Radiochromic film was used to verify relative dosimetry. 9 week old C57BL/6J mice were assigned into groups of 20 to receive 45 Gy to the hind leg. We investigated radiation-induced acute and late damage on the skin and mesenchymal tissues, such as skin toxicity, neutrophil activation, release of TGFβ-1 and development of lymphedema.
Results: The FLASH group showed no difference from Standard group in skin reaction, percent weight change, and skin inflammation at Day 18. The IVIS imaging of the released myeloperoxidase activity by the activated neutrophils gave a similar result at 9, 18, 27, 36, and 72 days post-irradiation. Late effects, such as lymphedema, showed no difference between groups as well.
Conclusion: Under these conditions, there was no significant difference in the severity of skin toxicity and other manifestations of radiation-induced injury. This could potentially be attributed to either the high dose used or to the fact that photons may not exhibit FLASH effects at such dose rates ( ≥ 40 Gy/s), unlike electrons or protons. A higher threshold dose rate might be needed for X-rays. More research is required to delineate these two possibilities.